CPC9909

R03

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7

2.3 Input Voltage Regulator

The CPC9909 has an internal voltage regulator that

pin is the internal voltage regulator output pin and

must be bypassed by a low-ESR capacitor to provide

a low impedance path for high frequency switching

noise.

The CPC9909 driver does not require the bulky

start-up resistors typically needed for off-line

controllers. The internal voltage regulator provides

sufficient voltage and current to power internal IC

and can be used as a bias voltage for external

circuitry.

The internal voltage regulator can be bypassed by

slightly higher than the internally generated regulator

voltage. This reduces the power dissipation of the

integrated circuit, and it is more suitable in isolated

applications where an auxiliary winding can be used to

to the quiescent current drawn by the internal circuitry

(which is specified at 0.6mA maximum) plus the gate

driver current. See “Shut-Down Mode Supply Current”

in Section 1.5 “Electrical Characteristics” on

page 5.

The current draw of the gate driver depends on the

switching frequency and the gate charge of the

external power MOSFET. The total input current can

be calculated by:

2.4 Current Sense Resistor

The peak LED current is set by an external sense

ground.

The value of the current sense resistor is calculated

based on the average LED current desired, the current

sense threshold, and the inductor ripple current.

The peak-to-peak difference in the inductor current

waveform is referred to as inductor ripple current (the

inductor is typically selected to be large enough to

keep this ripple within 30% of the average). Factor in

the ripple current when calculating the sense resistor.

The current sense resistor value can be found by:

Where:

•

I

Combining terms:

2.5 Current Sense Blanking

The CPC9909 has an internal current sense blanking

circuit. When the power MOSFET is turned on, the

external inductor can cause an undesired spike at the

current sense pin, initiating a premature termination of

the gate pulse. To avoid this condition, a typical 400ns

internal leading edge blanking time is implemented,

thereby eliminating the need for external RC filtering,

and simplifying the design. During the current sense

blanking time, the current limit comparator is disabled,

preventing the gate-drive circuit from terminating the

gate-drive signal.

2.6 Enable/Disable Function

driver. Connecting PWMD to GND disables the gate

driver and sets the device in standby mode. In standby

mode, the quiescent current is 0.6mA maximum.

2.7 Minimum Off-Time One-Shot

The CPC9909 uses a fixed off-time control scheme.

The minimum off-time is set by an external resistor

connected between the RT and GND terminals.

The off-time can be determined by:

Off-time selection indirectly determines the switching

frequency of the LED driver.

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=

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 66

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 0.8

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